1. Technical Field
The present invention relates to cubic boron nitride-base sintered ceramics for cutting tool. The sintered ceramics is such that the ceramics do not suffer cutting edge damage and chipping even when used under severe conditions such as high speed or high feed rate interrupted cutting of hardened steels, and their cutting edge exhibits an excellent resistance to wear.
2. Background Art
Cubic boron nitride (denoted as c-BN below)-based ceramics sintered under high pressure and high temperature are known. For example publication of unexamined J.P. Patent Application Laid Open No. 55-62862 discloses a c-BN ceramics sintered under high pressure and having 10% to 60% by volume of one or more of titanium carbide, titanium nitride, and titanium carbonitride (denoted hereunder as TiC, TiN and TiCN) as a bonding phase, 0.1% to 18% by volume of aluminum oxide (denoted hereunder as Al.sub.2 O.sub.3) as a bonding phase, and the remainder of c-BN as a dispersed phase, and unavoidable impurities (note: in the above and the following, percentage refers to percentage by volume).
Furthermore, the use of cutting tips formed from this high pressure sintered ceramics for finishing cutting of ceramics such as hardened steels having a Rockwell hardness (C scale) of 55-62, and for finish cutting of materials such as nickel or cobalt based super alloys is well known.
On the other hand, in recent years the increase in the performance of cutting machines has resulted in remarkable labor savings together with a trend toward higher cutting speeds and heavier cutting. With the beforementioned cutting tips however made from c-BN high pressure sintered ceramics, the strength of these tips is inadequate. Especially for high speed interrupted cutting of high strength steels such as case carborized hardened steels and high frequency hardened steels, or for cutting under severe conditions such as high feed interrupted cutting. Moreover, due to this lack of strength, the cutting edge of the cutting tip is susceptible to cracking and chipping resulting in unsatisfactory cutting performance.
The present inventors have studied the beforementioned conventional cutting tips manufactured from c-BN ceramics sintered under high pressure, from the view point of the above situation and has conducted experiments aimed at improving the strength. Results (1), (2) and (3) of these experiments are given below.
(1) With the conventional cutting tip manufactured from c-BN high pressure sintered ceramics, it was found that the crystal grain sizes of one or more of the TiC, TiN, TiCN and Al.sub.2 O.sub.3 forming the bonding phase exceeded a maximum grain size of 3 microns. The reason for the large crystal grain size was considered due to the fact that even when manufacturing the cutting tip using minute grain sizes of less than 1 micron for the raw material powder, grain growth occurred during the high pressure sintering. Consequently, with the existence of crystal grain sizes in the bonding phase exceeding a maximum grain size of 3 microns, sufficient strength could not be maintained.
(2) In the above case, it was ascertained that even though grain growth occurred in the bonding phase, the high pressure sintering process had practically no influence on the crystal grain size of c-BN. Accordingly, if a c-BN powder having a crystal grain size of not more than 1 micron was used for the raw material powder, the c-BN crystal grain size remained below 1 micron even after the high pressure sintering process.
(3) On the other hand, when using one or more of titanium and aluminum nitride, carbide or carbonitride compounds (denoted hereunder as Ti.sub.2-3 AlN, Ti.sub.2-3 AlC, and Ti.sub.2-3 AlCN respectively) with an oxygen content preferably in the range from 8% to 20% by weight as a raw material powder for the bonding phase, these compounds appear to decompose during the high pressure sintering process and react with the c-BN.
In this reaction one or more of TiC, TiN and TiCN, one or more of Al.sub.2 O.sub.3 and aluminum nitride (denoted hereunder as AlN), and titanium boride (denoted hereunder as TiB.sub.2) is formed, these compounds making up the bonding phase, and having extremely fine crystal grain sizes of not more than 1 micron. At the same time, the c-BN high pressure sintered ceramics is formed with the c-BN dispersed phase having a crystal grain size of less than 1 micron. The above noted results (1), (2) and (3) of these experiments were known by the inventors but not to the public.
By making the cutting tips from the above mentioned ceramics, the cutting tip is attributed with an extremely high strength, so that even when cutting under severe conditions, damage and chipping (micro crack) of the cutting edge of the cutting tip does not occur. Furthermore, the cutting tip is found to exhibit excellent wear resistance.